An anonymous letter referring to a gun in the Mark Duggan case has surfaced, a pre-inquest hearing has been told.

Ashley Underwood QC, counsel for the inquest which is set to begin on September 16, said it is a line of investigation that "we are chasing down".

He told a pre-inquest hearing at the Royal Courts of Justice: "An anonymous letter has come to light. It makes an allegation about how it was Mark Duggan came by the gun, if indeed Mark Duggan had a gun, and was it a put-up job."

Mr Duggan, 29, died when he was shot by police who stopped the taxi in which he was travelling in Tottenham, north London, in August 2011. There has been speculation that he was on his way to carry out a revenge killing for his cousin's death.

Mr Duggan's death is seen as having led to the riots which broke out in the following days and spread to other areas of the capital and to other cities.

A number of police officers who are due to be witnesses at the inquest have applied to remain anonymous.

Questions were also raised over a request for a live link to be set up so police officers could hear the evidence in a different room during the inquest.

Leslie Thomas, for Mr Duggan's family, argued against a situation which is "uncontrolled" and allows officers to sit "in a secret room" somewhere in the court building. If it is to be allowed, court officials should be nearby to monitor the situation, he suggested.

He said: "Given the grave strength of feeling of this case, we say this is sending the wrong message."

Hugo Keith, for the Metropolitan Police, stressed there is no suggestion that anything inappropriate would happen and noted that they were entitled to hear what is happening in court.